For some diesel engine applications, the use of a switching type-1 (direct acting cam on bucket tappet) DOHC valvetrain is desired as a high speed valvetrain with variable swirl capabilities (e.g., via tappet switching). In embodiments, to fit the DOHC configuration with a centered, vertical HPCR fuel injector, the engine valves may be splayed. In some conventional diesel engines, the combustion chamber is defined almost entirely within a piston bowl defined in the piston. As a result, splaying the engine valves can create a volume of dead space at each in-cylinder engine valve seat. The dead space associated with intake valves may shroud the incoming intake charge, diminishing cylinder filling efficiency; and the dead space associated with the exhaust valves may shroud the exhaust gas, diminishing cylinder emptying efficiency. To mitigate the effects of the resulting shrouding, many diesel engines include a cylinder head having a semi-hemispherical lower surface, which may facilitate a lower pressure drop across the engine valves and better engine breathing (i.e., higher volumetric efficiency).
To facilitate various squish characteristics, the prior art includes a number of piston designs that incorporate a dome-shaped crown and a number of cavities or pockets defining the combustion chamber. For example, U.S. Pat. No. 2,644,433, to Anderson, discloses a combustion chamber defined by twin diametrically opposed cavities defined in a piston crown, which is domed-shaped to correspond to a spherical recess in the bottom face of the cylinder head. U.S. Pat. No. 5,285,755, to Regueiro, discloses a piston head that may be dome-shaped to correspond to a pentroof angle of the fire deck of the cylinder head and that includes four recessed lobes in its top surface. In further advances, axi-symmetric piston bowls having flat upper surfaces were designed to mitigate production of unwanted particulates and excesses of unburned hydrocarbons. For example, U.S. Pat. No. 8,276,563, to Quigley et al., discloses a piston having a flat upper surface and an axi-symmetric combustion bowl with a substantially circular shape. U.S. Publication No. 2012/0234285 discloses a piston bowl having a flat upper surface, an axi-symmetric combustion bowl, and a number of valve pockets arranged about the periphery of the piston crown.
Previously, manufacturers of diesel engines have steered away from pistons having semi-hemispherical crowns and axi-symmetric combustion bowls because it was believed that this shape would be conducive to excess unburned hydrocarbons and creation of excessive particulate matter. The inventors of the present disclosure have discovered that embodiments of the design described herein do not result in excess unburned hydrocarbons and creation of excessive particulate matter.